Methods and systems for reducing particulate deposition on photomask

ABSTRACT

Particulate deposition rate on a photolithographic mask, particularly of tin (Sn) particles produced within an EUV light source, is reduced by producing turbulence within a radiation source chamber of the EUV light source. Turbulence can be produced by changing the temperature, pressure, and/or gas flow rate within the radiation source chamber. The turbulence reduces the number of particles exiting the EUV light source which could be deposited on the photomask.

PRIORITY CLAIM AND CROSS-REFERENCE

This application claims the benefit of U.S. Provisional PatentApplication Ser. No. 63/172,956, filed Apr. 9, 2021, which isincorporated herein by reference in its entirety.

BACKGROUND

A photolithographic patterning process uses a reticle (i.e. photomask)that includes a desired mask pattern. The reticle may be a reflectivemask or a transmission mask. In the process, ultraviolet light isreflected off the surface of the reticle (for a reflective mask) ortransmitted through the reticle (for a transmission mask) to transferthe pattern to a photoresist on a semiconductor wafer. The minimumfeature size of the pattern is limited by the light wavelength. Deepultraviolet (UV) lithography uses a wavelength of 193 nm or 248 nm.Extreme ultraviolet (EUV) light, which spans wavelengths from 124nanometers (nm) down to 10 nm, is currently being used to provide smallminimum feature sizes. At such short wavelengths, particle contaminantson the photomask can cause defects in the transferred pattern.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1 is a schematic view of a lithography system, in accordance withsome embodiments.

FIG. 2 is a method in accordance with some embodiments.

FIG. 3A is an illustrative graph showing an environmental parameterchanging over irregular time periods, in accordance with someembodiments.

FIG. 3B is an illustrative graph showing an environmental parameterchanging over regular time periods, in accordance with some embodiments.

FIG. 3C is an illustrative graph showing two environmental parametersbeing independently changed in both their time period and magnitude ofchange, in accordance with some embodiments.

FIG. 4 is a cross-sectional view of an example mask and pellicleassembly, which is protected by use of the methods of the presentdisclosure, in accordance with some embodiments.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the provided subjectmatter. Specific examples of components and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and are not intended to be limiting. For example, the formationof a first feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

Numerical values in the specification and claims of this applicationshould be understood to include numerical values which are the same whenreduced to the same number of significant figures and numerical valueswhich differ from the stated value by less than the experimental errorof conventional measurement technique of the type described in thepresent application to determine the value. All ranges disclosed hereinare inclusive of the recited endpoint.

The term “about” can be used to include any numerical value that canvary without changing the basic function of that value. When used with arange, “about” also discloses the range defined by the absolute valuesof the two endpoints, e.g. “about 2 to about 4” also discloses the range“from 2 to 4.” The term “about” may refer to plus or minus 10% of theindicated number.

The present disclosure may refer to temperatures for certain methodsteps. It is noted that these references are to the temperature at whichthe heat source is set, and do not specifically refer to the temperaturewhich must be attained by a particular material being exposed to theheat.

In the present disclosure, the terms “photolithographic mask”, “mask”,“photomask”, and “reticle” are used interchangeably.

In the present disclosure, the terms “particulate”, “residualparticles”, and “residual debris” are used interchangeably.

The term “optic” is used herein to refer broadly to components whichreflect and/or transmit and/or operate on incident light, such asmirrors, lenses, windows, filters, or reflectors. Such components canoperate at different wavelengths from each other.

The term “turbulence” or “turbulent” is used herein to refer to unsteadyor chaotic particle motion, in contrast to laminar flow or Brownianmotion.

The present disclosure is generally related to extreme ultraviolet (EUV)lithography systems and methods. The EUV lithography process employslight in the extreme ultraviolet (EUV) region, having a wavelength ofabout 1 nm to about 100 nm. One type of EUV light source islaser-produced plasma (LPP). LPP technology produces EUV light byfocusing a high-power laser beam onto small target droplets to formhighly ionized plasma that emits EUV radiation with a peak maximumemission at about 13.5 nm. The EUV light is then collected by acollector mirror and reflected by optics towards a photolithographicmask and then towards a lithography target, e.g., a wafer.

More particularly, the present disclosure is related to systems andmethods for mitigating tin (Sn) deposition on an EUV photolithographicmask which can result from a laser produced plasma (LPP) EUV radiationsource. In this regard, at short wavelengths, particle contaminants onthe photomask can cause defects in the transferred pattern. Sometimes, apellicle mounted over the photomask can be used to protect the photomaskfrom contaminating particles. Any contaminating particles which land onthe pellicle membrane are kept out of the focal plane of the photomask,thus reducing or preventing defects in the transferred pattern caused bythe contaminating particles. However, for EUV applications, it may bedesirable to eliminate the pellicle to reduce EUV absorption. It wouldthus be desirable to reduce contaminant particle levels overall. In thisregard, the tin (Sn) particles used to generate EUV light can escapefrom the radiation source chamber of the EUV light source and enter theprocess chamber containing the photomask itself.

Generally, then particulates that fall onto or are deposited upon themask, such as tin (Sn) particles, can easily damage the mask pattern andresult in heavy yield loss. A high particulate fall-on or depositionrate will require mask repair and cause production line instability(i.e. downtime). Accordingly, the present disclosure desires to reducethe particulate fall-on deposition rate on the mask, by reducing thelevel of particulates (such as tin particles) which coming from the EUVlight source and the radiation source chamber thereof.

FIG. 1 is a schematic diagram, not drawn to scale, illustrating thevarious components of an EUV lithography system 100. Generally, the EUVlithography system includes an EUV light source 110 that generates EUVlight and a laser source 120. Downstream of the EUV light source is anillumination stage 130, which collects and focuses the EUV light on thephotomask 140. Downstream of the photomask 140 is the projection opticsmodule 150, which is configured for imaging the pattern of the photomaskonto a substrate 160, such as a semiconductor wafer. The lithographysystem can include other modules or be integrated with or coupled toother modules.

The EUV light source 110 itself includes a radiation source chamber 112which encloses the plasma reaction that creates EUV light. The radiationsource chamber includes a target droplet generator 114. The targetdroplet generator deposits a plurality of target droplets 116 into theradiation source chamber. In some embodiments, the target droplets aretin (Sn) droplets. In some embodiments the target droplets are made oftin (Sn). In other embodiments, the target droplets may includealternative types of material, for example, a tin-containing liquidmaterial such as eutectic alloy containing tin or lithium (Li). The tindroplets may be deposited at a rate, for example of about 50 thousanddroplets per second.

A laser source 120 is also present, which emits one or more laser beamsinto the radiation source chamber that contact the target droplets toproduce EUV light. In some embodiments, the laser source may be a carbondioxide (CO₂)or a neodymium-doped yttrium aluminum garnet (Nd:YAG) lasersource, The laser source can be a multi-stage laser having a pluralityof stages configured to amplify laser light produced by a prior stage.In some embodiments, the laser source can produce multiple beams. Forexample, as illustrated here, a pre-heat laser pulse 122 can be used tocreate a low-density target plume, which is subsequently heated (orreheated) by a main laser pulse 124 that generates EUV light. The lasersource generally also includes optics and other focal components fordirecting the laser beams in a desired direction.

The radiation source chamber 112 includes windows or lenses, which aresubstantially transparent to the laser wavelength. The generation of thelaser beams is synchronized with the target droplets. The pre-heat laserpulse 122 heats the target droplets and expands them into lower-densitytarget plumes. A delay between the pre-pulse 122 and the main pulse 124is controlled to allow the target plume to form and to expand to anoptimal size and geometry. When the main pulse 124 heats the targetplume, a high-temperature plasma field 125 is generated. The plasmaemits EUV radiation in all directions. The EUV light travelling in thewrong direction (i.e. away from the output port) is collected by acollector mirror 176.

The collector mirror 176 has a reflective surface that reflects andfocuses EUV light. In some embodiments, the collector mirror has anellipsoidal shape. The collector mirror may be coated with materialssimilar to that of the photomask for reflecting EUV light. EUV light isthen directed towards an output port 119, through which the EUV lightexits the EUV light source to enter the rest of the lithography system.

A droplet catcher 118 may be installed opposite the target dropletgenerator 114. The droplet catcher is used to catch excess targetdroplets.

In the EUV light source, the plasma caused by the laser applicationcreates EUV light and also physical debris, such as ions, gases andatoms of the droplet. To prevent the accumulation of particulate anddebris on the collector mirror 176, a buffer gas can be injected alongthe surface of the collector mirror via gas ports 170. The buffer gascan be H₂, He, Ar, N₂, or some other inert gas, but is usually hydrogen.Hydrogen gas has a low EUV absorption profile. Hydrogen will react withmetals to form a metal hydride. In particular, hydrogen will react withtin to form SnH₄ as a gaseous product at the temperatures of the EUVgeneration process, which can be captured and pumped out. The gaseousSnH₄ is then pumped out of the radiation source chamber.

Other debris collection mechanisms may also be present. For example, tofurther trap residual tin particles and other debris, the interior ofthe radiation source chamber may include a plurality of vanes 172disposed around a frustoconical support frame which narrows in diameterfrom the plasma field to the output port. The vanes extend generallyradially inwards from the support frame. The vanes may be made, forexample, from molybdenum or stainless steel. The temperature of thevanes may be controlled to keep tin in a liquid state but not a gaseousstate, or in other words above about 231° C. but below about 1,100° C.(at the operating pressure of the EUV light source). For example, thevanes may include channels through which a liquid or gas can be flowed.The liquid or gas may be heated or cooled, as appropriate, to obtain adesired temperature for the vanes. The vanes may also be shaped todirect the liquid tin to an appropriate drain, for example via capillaryaction and/or wicking action.

Because gas molecules absorb EUV light, the radiation source chamber istypically maintained at vacuum or a low-pressure environment to avoidEUV intensity loss.

Continuing, in the illumination stage 130, the EUV light may becollected and focused as a beam, for example using field facet mirror132 that splits the beam into a plurality of light channels. These lightchannels can then directed using one or more relay mirrors 134 onto theplane of the photomask.

The projection optics module 150 may include refractive optics orreflective optics for carrying the image of the pattern defined by thephotomask. Illustrative mirrors 152, 154 are shown. The pattern is thenfocused onto substrate 160, which may be for example a silicon wafer.

FIG. 2 illustrates an exemplary method M in accordance with someembodiments, for operating an EUV light source. The method M may beimplemented, in whole or in part, by a system for use with extremeultraviolet (EUV) lithography or other lithography processes requiringlight that is generated by ionizing atoms and which results in residualparticles or droplets that could potentially be deposited on a mask.Additional operations can be provided before, during, and after themethod M, and some operations described can be replaced, eliminated,modified, moved around, or relocated for additional embodiments of themethod. One of ordinary skill in the art may recognize other examples ofsemiconductor fabrication processes that may benefit from aspects of thepresent disclosure. The method M is an example and is not intended tolimit the present disclosure beyond what is explicitly recited in theclaims. The method M is described below along with the referencenumerals of FIG. 1.

Referring now to FIG. 2, the method M begins at step S202, where aplurality of target droplets 116 is deposited into the radiation sourcechamber 112 by the target droplet generator. Next, at step S204, a lasercontacts the target droplets, thereby generating a plasma 125 that emitsEUV radiation in the radiation source chamber. This plasma generationalso produces residual debris or particles 101. The residual debris ofthe EUV radiation may include metal dust, target material vapor andmicro-droplets or clusters. The residual debris can be in several forms.For example, tin may be present as pure tin or a tin compound, e.g.,SnBr₄, SnH₄, etc., and/or may include oxides. Dust and othercontaminants may also be present in the radiation source chamber. Thisresidual particulate/debris can escape the radiation source chamber viathe output port 119 of the EUV light source and into the process chamber142 containing the photomask 140, where undesired particles canpotentially be deposited onto the photomask, for example on its surface,and thus damage the photomask.

Continuing, the method M proceeds to step S206, where turbulence iscreated in the radiation source chamber by adjusting one or more ofseveral tool parameters to be irregular over time. In some embodiments,the tool parameters may include temperature, pressure, or gas flow rate.In other embodiments, the tool parameters may include otherenvironmental and nonenvironmental parameters as well. The time periodof the changing parameters can be regular or irregular over differenttime periods. The magnitude of the changing parameters can also beregular or irregular. In some embodiments, the changed parameters are acombination of at least two of the tool parameters (temperature,pressure, or gas flow rate).

Altering at least one tool parameter in the radiation source chamberincreases turbulence in the radiation source chamber. Without thisturbulence in the radiation source chamber, the residual debris in theradiation source chamber could escape the radiation source chamber andend up being deposited on downstream optics, the pellicle, or thephotomask, as well as on other surfaces of the radiation source chamberitself. This degradation of the photomask and pellicle results inincreased down-time and lower product yield while the mask is repaired.The accumulation of residual debris on the photomask is reduced by theincrease in turbulence. As turbulence increases, escape of the residualdebris from the radiation source chamber (resulting from generation ofEUV radiation) is reduced.

As indicated in step S208 of method M, due to the increased turbulencein the radiation source chamber 112, the residual debris 101 is divertedaway from the output port 119, and instead collected into a collectionsource. In some embodiments, the collection source is a scrubber 174. Inother embodiments, the collection source can be some other means forcollection of particulates and debris.

Finally, method M proceeds to step S210 where the EUV radiation istransmitted towards the photomask 140. The particulate deposition rateon the photomask, particularly of tin (Sn) particles, is greatly reduceddue to the decreased particulate level in the radiation source chamber,which can no longer escape into the process chamber 142 containing thephotomask.

In some embodiments, the method M (illustrated in FIG. 2) may beimplemented in a computer program product that may be executed on acomputer. The computer program product may comprise a non-transitorycomputer-readable recording medium on which a control program isrecorded (stored), such as a disk, hard drive, or the like. Common formsof non-transitory computer-readable media include, for example, floppydisks, flexible disks, hard disks, magnetic tape, or any other magneticstorage medium, CD-ROM, DVD, or any other optical medium, a RAM, a PROM,an EPROM, a FLASH-EPROM, or other memory chip or cartridge, or any othertangible medium from which a computer can read and use.

Alternatively, the method may be implemented in transitory media, suchas a transmittable carrier wave in which the control program is embodiedas a data signal using transmission media, such as acoustic or lightwaves, such as those generated during radio wave and infrared datacommunications, and the like.

Further, the exemplary system and method may be implemented on one ormore general purpose computers, special purpose computer(s), aprogrammed microprocessor or microcontroller and peripheral integratedcircuit elements, an ASIC or other integrated circuit, a digital signalprocessor, a hardwired electronic or logic circuit such as a discreteelement circuit, a programmable logic device such as a PLD, PLA, FPGA,Graphical card CPU (GPU), or PAL, or the like. In general, any device,capable of implementing a finite state machine that is in turn capableof implementing the functions and structures described above (and/orembodied in the flowchart shown in FIG. 2) can be used to implement themethods for reducing particulate deposition rate on a photolithographicmask by producing turbulence within a radiation source chamber.

As discussed above, turbulence in the radiation source chamber isgenerated by changing one or more of several tool parameters to beirregular over time. In some embodiments, the changed tool parametersmay include temperature, pressure, or gas flow rate. As illustrated inFIG. 1, these three parameters may be monitored using sensors, such asthermometer, pressure gauge, and/or flow meter, which feed tocontroller. The tool parameters may be changed by altering theirsetpoints using the controller.

Turbulence in the radiation source chamber is increased as theenvironmental parameters are changed over irregular or regular timeperiods. When changing irregularly, four consecutive time periods atdifferent setpoints are considered. If the four time periods are not thesame, then the environmental parameter is being changed at irregulartime periods. If the four time periods are the same, then theenvironmental parameter is being changed at regular time periods.

FIG. 3A is an example of a temperature versus time graph where thetemperature of the radiation source chamber is altered over irregulartime periods. As illustrated here, the controller will vary thetemperature between relatively high temperature T₁ and relatively lowtemperature T₂. Two different time periods are seen here: (1) a hightemperature time period during which the temperature is maintained athigh temperature T₁; and (2) a low temperature time period during whichthe temperature is maintained at low temperature T₂. High temperaturetime periods are indicated here as TP₁ (t₁₋₀), TP₃ (t₃-t₂), and TP₅(t₅-t₄). Similarly, low temperature time periods are indicated here asTP₂ (t₂-t₁), TP₄ (t₄-t₃), and TP₆ (t₆-t₅).

As illustrated here, the length of time periods TP₁, TP₂, TP₃, and TP₄are not all the same. Thus, the temperature here is being changed overirregular time periods. This observation would also hold for any otherfour consecutive time periods illustrated here.

It is also noted that TP₄ has a different setpoint from prior periodTP₃, which has a different setpoint from prior period TP₂, which has adifferent setpoint from prior period TP₄. Only consecutive setpointsmust be different, the four time periods do not have to have completelydifferent setpoints.

FIG. 3B is an example of a temperature versus time graph where thetemperature of the radiation source chamber is altered over regular timeperiods. As illustrated here, the length of time periods TP₁, TP₂, TP₃,and TP₄ are all the same. Thus, the temperature here is being changedover irregular time periods.

It is noted that it is possible for three time periods to be the same,but if the fourth time period is different, then the environmentalparameter is still being changed over irregular time periods. Thus, itis contemplated that most of the time, the environmental parameter(s) isbeing changed over irregular time periods.

The direction in which the environmental parameter is changed is notparticularly significant. In a first embodiment, the change in anenvironmental parameter moves between a relatively low setpoint and arelatively a relatively high setpoint over regular time periods. In asecond embodiment, the change in an environmental parameter willtransition between a relatively high setpoint and a relatively lowsetpoint over regular time periods. In a third embodiment, the change inan environmental parameter transitions between a relatively low setpointand a relatively high setpoint over irregular time periods. In a fourthembodiment, the change in an environmental parameter transitions betweena relatively high setpoint and a relatively low setpoint over irregulartime periods. It is contemplated that varying between two setpoints, arelatively high setpoint and a relatively low setpoint, will besufficient to create turbulence. However, varying between more than twosetpoints (e.g. three or four setpoints) is contemplated as well inaccordance with some embodiments.

In embodiments where the temperature is varied, the temperature of theradiation source chamber can be as low as −20° C. and as high as 600° C.However, generally speaking, if one temperature setpoint is below themelting point of tin (˜231° C.), then the other temperature setpointshould be above the melting point of tin. It is also noted that thetemperature can be changed between setpoints relatively quickly. Forexample, it may take only 10 minutes to change the temperature from 50°C. to 500° C. In some embodiments, the difference between first andsecond temperature setpoints is at least 50° C., or at least 100° C. Thedifference between the first and second setpoints may be at most 200° C.

In a first embodiment, the high temperature setpoint T₁ can be fromabout 200° C. to about 600° C., and the low temperature setpoint T₂ canbe from about 10° C. to about −20° C.

In a second embodiment, the high temperature setpoint T₁ can be fromabout 350° C. to about 600° C., and the low temperature setpoint T₂ canbe from about 230° C. to about 300° C. It is noted that both setpointsare close to or above the melting point of tin.

In a third embodiment, the high temperature setpoint T₁ can be fromabout 250° C. to about 500° C., and the low temperature setpoint T₂ canbe from about 50° C. to about 200° C.

In a fourth embodiment, the difference between the high temperaturesetpoint T₁ and the low temperature setpoint T₂ is at least 50° C., andboth temperature setpoints are above 230° C.

The temperature can be changed between first and second setpoints suchthat each time period ranges from about 1 hour to about 96 hours (4days). In more particular embodiments, each time period ranges fromabout 4 hours to about 72 hours, or from about 24 hours to about 96hours, or from about 24 hours to about 48 hours (about 1 day to about 2days), or from about 48 hours to about 72 hours (about 2 days to about 3days). For purposes of monitoring the temperature, the temperature ofthe radiation source chamber can be measured at the vanes (it isrecognized that, for example, the temperature of the chamber may be verydifferent where the plasma is located). Referring to FIG. 1, thetemperature of the radiation source chamber can be changed, for example,by flowing liquid/gas of different temperature through the vanes or bycooling any gases entering the chamber (for example through gas ports).In some embodiments, only the temperature is changed to createturbulence, while the pressure and gas flow rate are maintained.

With respect to pressure, as previously noted, the radiation sourcechamber in the EUV light source is typically maintained at vacuum or alow pressure (e.g. 0.001 Pa) to reduce EUV absorption by gas molecules.However, it is believed the pressure in the radiation source chambercould potentially be as high as 101 kPa (i.e. 1 atmosphere) and stillobtain the desired EUV output.

Thus, when the pressure is varied, the pressure of the radiation sourcechamber may be as low as 0 pascals (Pa) and as high as 101 kPa. Thepressure can be changed between setpoints relatively quickly. Forexample, it may take only 30 minutes to change the pressure between twosetpoints. In some embodiments, the difference between first and secondpressure setpoints is at least 10 pascals (Pa), or at least 20 Pa, or atleast 50 Pa. The difference between the setpoints may be at most 500 Pa,or at most 200 Pa, or at most 100 Pa.

In a first embodiment, the high pressure setpoint P₁ can be from about50 Pa to about 50,000 Pa, and the low pressure setpoint P₂ can be fromabout 0 Pa to about 10 Pa.

In a second embodiment, the high pressure setpoint P₁ can be from about100 Pa to about 1000 Pa, and the low pressure setpoint P₂ can be fromabout 0 Pa to about 10 Pa.

In a third embodiment, the high pressure setpoint P₁ can be from about50 Pa to about 100 Pa, and the low pressure setpoint P₂ can be fromabout 0 Pa to about 10 Pa.

In a fourth embodiment, the difference between the high pressuresetpoint P₁ and the low pressure setpoint P₂ is at least 10 Pa, and bothpressure setpoints are above 0 Pa.

The pressure can be changed between first and second setpoints such thateach time period ranges from about 1 hour to about 96 hours (4 days). Inmore particular embodiments, each time period ranges from about 4 hoursto about 72 hours, or from about 24 hours to about 96 hours, or fromabout 24 hours to about 48 hours (about 1 day to about 2 days), or fromabout 48 hours to about 72 hours (about 2 days to about 3 days).Referring to FIG. 1, the pressure of the radiation source chamber can bechanged, for example, by changing the pump rate of vacuum pump or byadding a gas into the chamber. In some embodiments, only the pressure ischanged to create turbulence, while the temperature and gas flow rateare maintained.

With respect to gas flow rate, as previously noted, some gases areprovided into the radiation source chamber. For example, gases such ashydrogen gas (e.g., H₂), He, Ar, N₂, or other inert gases are providedto serve as buffers, for example to reduce particulate buildup on thecollector mirror, or for particulate capture/control or for otherpurposes. A buffer gas, such as hydrogen gas, normally flows into theradiation source chamber at a rate of 1 standard liter per minute (slm).Extra dry clean air normally flows into the radiation source chamber ata rate of ˜10 slm.

When the gas flow rate is varied, only the flow of a particular gas(i.e. the target gas) is considered. The gas flow rate can be changedbetween setpoints quickly. For example, it may take only 10 minutes tochange the gas flow rate between two setpoints. In some embodiments, thedifference between first and second gas flow rate setpoints is at least0.5 slm, or at least 1 slm. The difference between the setpoints may beat most 0.8 slm, or at most 1.0 slm, or at most 2 slm.

In a first embodiment, the high gas flow rate setpoint F₁ for hydrogencan be from about 0.5 slm to about 1 slm, and the low gas flow ratesetpoint F₂ for hydrogen can be from about 0.1 slm to about 0.5 slm. Thesetpoint F₁ is greater than the setpoint F₂.

In a second embodiment, the difference between the high gas flow ratesetpoint F₁ for hydrogen and the low gas flow rate setpoint F₂ forhydrogen is at least 0.5 slm, and both gas flow rate setpoints are above0.1 slm.

In a third embodiment, the high gas flow rate setpoint F₁ for extra dryclean air (EDCA) can be from about 5 slm to about 10 slm, and the lowgas flow rate setpoint F₂ for EDCA can be from about 0.1 slm to about 5slm. The setpoint F₁ is greater than the setpoint F₂.

In a fourth embodiment, the high gas flow rate setpoint F₁ for EDCA canbe from about 8 slm to about 10 slm, and the low gas flow rate setpointF₂ for EDCA can be from about 0 slm to about 6 slm.

In a fifth embodiment, the difference between the high gas flow ratesetpoint F₁ for EDCA and the low gas flow rate setpoint F₂ for EDCA isat least 0.5 slm, and both gas flow rate setpoints are above 0 slm.

The gas flow rate can be changed between first and second setpoints suchthat each time period ranges from about 1 hour to about 96 hours (4days). In more particular embodiments, each time period ranges fromabout 4 hours to about 72 hours, or from about 24 hours to about 96hours, or from about 24 hours to about 48 hours (about 1 day to about 2days), or from about 48 hours to about 72 hours (about 2 days to about 3days). Referring to FIG. 1, the gas flow rate of the radiation sourcechamber can be changed, for example, by changing the flow rate throughgas ports. In some embodiments, only the gas flow rate is changed tocreate turbulence, while the temperature and pressure are maintained.

In another exemplary embodiment, two environmental parameters arealtered to generate turbulence in the radiation source chamber. Forexample, the temperature and the pressure, or the temperature and thegas flow rate, or the pressure and the gas flow rate can be altered.These parameters can be changed as described above. It is noted that thetwo parameters do not have to be changed at the same time, or for thesame time periods, though that might be more convenient. For example,FIG. 3C illustrates a system where the temperature and the pressure ofthe radiation source chamber are altered to generate turbulence. Asillustrated here, the length of temperature time periods TTP₁, TTP₂,TTP₃, TTP₄, TTP₅, and TTP₆ are different, or in other words thetemperature is changing over irregular time periods. In contrast thelength of pressure time periods PTP₁, PTP₂, PTP₃, PTP₄, PTP₅, and PTP₆are the same, so that the temperature is changing over regular timeperiods. In addition, the times at which the temperature or pressure ischanging differ from each other.

In another exemplary embodiment, the temperature, the pressure, and thegas flow rate are all altered to generate turbulence in the radiationsource chamber. Again, these parameters can be changed as describedabove, and they do not have to be changed at the same time, or for thesame time periods, though that might be more convenient.

Referring back to FIG. 1, the environmental parameters can be changed bya controller operating a computer program which alters the setpoints.For example, controller is shown as being able to alter the temperatureof vanes, or the pump rate of vacuum pump, or the gas flow rate throughgas ports. The controller can randomly vary the time periods and themagnitudes of the setpoints within specified ranges, or can follow a setschedule for changing the setpoints of a given environmental parameter.The controller may include a user interface for communicating withoperators.

The turbulence created by repeatedly changing the environmentalparameters creates additional air flow patterns which causeparticulates, such as tin particles, to be diverted away from the outputport and, for example, causes the particulates to be diverted to aparticle trap, such as air scrubber. As a result, such particulates arenot able to escape from the EUV light source into the processing chamberwhere the particulates might land on the photomask. It is believed thatthe methods of the present disclosure can reduce the number of tinparticles being deposited on the photomask by up to 50%.

FIG. 4 illustrates a cross-sectional view of an exemplary mask assembly305 useful in lithography, according to some embodiments, which isprotected by the methods of the present disclosure. The mask assembly305 includes a mask 300 and a pellicle assembly 320. The illustrativemask 300 (also referred to in the art as a mask, photomask, or similarphraseology) is a reflective mask of a type commonly used in EUVlithography, and includes a substrate 302, alternating reflective layers304 and spacing layers 306, a capping layer 308, an EUV absorbing layer310 that is patterned to define a mask pattern, an anti-reflectivecoating (ARC) 312, and a conductive backside layer 314. The illustrativemask 300 is merely a nonlimiting example. More generally, pellicles, asdisclosed herein, can be used with substantially any type of reflectiveor transmission mask. As another example (not shown), the mask may be atransmission mask, in which case the substrate is transmissive for lightat the wavelength at which the lithography is performed. In general, thereflective or transmissive mask includes a substrate (e.g., substrate302) and a mask pattern (e.g., absorbing layer 310) disposed on thesubstrate. The pellicle assembly 320 includes a mounting frame 322, anadhesive layer 324, and a pellicle membrane 330. In some non-limitingillustrative embodiments, the mask and pellicle assembly are intendedfor use with EUV light wavelengths, for example from 124 nm to 10 nm,including about 13.5 nm.

In embodiments, the substrate 302 is made from a low thermal expansionmaterial (LTEM), such as quartz or titania silicate glasses availablefrom Corning under the trademark ULE. This reduces or prevents warpingof the mask due to absorption of energy and consequent heating. Thereflective layers 304 and the spacing layers 306 cooperate to form aBragg reflector for reflecting EUV light. In some embodiments, thereflective layers may comprise molybdenum (Mo). In some embodiments, thespacing layers may comprise silicon (Si). The capping layer 308 is usedto protect the reflector formed from the reflective layers and thespacing layers, for example from oxidation. In some embodiments, thecapping layer comprises ruthenium (Ru). The EUV absorbing layer 310absorbs EUV wavelengths and is patterned with the desired pattern. Insome embodiments, the EUV absorbing layer comprises tantalum boronnitride. The anti-reflective coating (ARC) 312 further reducesreflection from the EUV absorbing layer. In some embodiments, theanti-reflective coating comprises oxidized tantalum boron nitride. Theconductive backside layer 314 permits mounting of the illustrative maskon an electrostatic chuck and temperature regulation of the mountedsubstrate 302. In some embodiments, the conductive backside layercomprises chrome nitride.

The mounting frame 322 supports the pellicle membrane at a heightsufficient to take the pellicle membrane 330 outside the focal plane ofthe lithography, e.g., several millimeters (mm) over the mask in somenonlimiting illustrative embodiments. The mounting frame itself can bemade from suitable materials such as anodized aluminum, stainless steel,plastic, silicon (Si), titanium, silicon dioxide, aluminum oxide(Al₂O₃), or titanium dioxide (TiO₂). Vent holes may be present in themounting frame for equalizing pressure on both sides of the pelliclemembrane.

The adhesive layer 324 is used to secure the pellicle membrane to themounting frame. Suitable adhesives may include a silicon, acrylic,epoxy, thermoplastic elastomer rubber, acrylic polymer or copolymer, orcombinations thereof. In some embodiments, the adhesive can have acrystalline and/or amorphous structure. In some embodiments, theadhesive can have a glass transition temperature (Tg) that is above amaximum operating temperature of the photolithography system, to preventthe adhesive from exceeding the Tg during operation of the system.

The systems and methods of the present disclosure attain the advantageof greatly reducing deposition of particulates, especially tinparticles, on the photomask. Reduced particulate deposition on the maskincreases the production and yield capacity of the overall lithographysystem by reducing the need for downtime and repair during theproduction process.

Another advantage of the present disclosure is that the methods reducethe need to use a pellicle due to reduced particulate levels. This mayincrease the EUV light dosage available for processing steps downstreamof the photomask.

Another advantage of the present disclosure is that the lowerparticulate levels may increase the longevity of the pellicle, if apellicle is used on the photomask.

Another advantage of the present disclosure is that the turbulencereduces the deposition of particulates in the radiation source chamber,which increases EUV productivity by increasing uptime due to reducedneed to clean the interior of the radiation source chamber itself.

Some embodiments of the present disclosure thus relate to a method forreducing particulate deposition rate on a photomask. Turbulence isproduced within a radiation source chamber. The turbulence reduces alevel of particulates exiting the radiation source chamber and enteringa processing chamber containing the photomask.

Other embodiments of the present disclosure relate to an EUV lightsource having means for changing at least one of the pressure,temperature, or gas flow rate in a radiation source chamber thereof. Acontroller is used to change one or more of these parameters at regularor irregular time periods.

Other embodiments of the present disclosure relate to a method forgenerating turbulence in a radiation source chamber of an EUV lightsource by changing at least one of the pressure, temperature, or gasflow rate in the radiation source chamber.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. A method for reducing particulate deposition rateon a photomask, comprising: producing turbulence within a radiationsource chamber, wherein the turbulence reduces a level of particulatesexiting the radiation source chamber and entering a processing chambercontaining the photomask.
 2. The method of claim 1, wherein theturbulence is produced by changing at least one of the temperature, thepressure, and the gas flow rate in the radiation source chamber.
 3. Themethod of claim 2, wherein the turbulence is produced by changing thetemperature in the radiation source chamber between a first temperaturesetpoint and a second temperature setpoint.
 4. The method of claim 3,wherein the first temperature setpoint and the second temperaturesetpoint differ by at least 50° C.
 5. The method of claim 4, wherein thefirst temperature setpoint and the second temperature setpoint are bothabove 230° C.
 6. The method of claim 3, wherein the first temperaturesetpoint is from about 200° C. to about 600° C., and the secondtemperature setpoint is from about 10° C. to about −20° C.
 7. The methodof claim 2, wherein the turbulence is produced by changing the pressurein the radiation source chamber between a first pressure setpoint and asecond pressure setpoint.
 8. The method of claim 9, wherein the firstpressure setpoint and the second pressure setpoint differ by at least 10pascals.
 9. The method of claim 9, wherein the first pressure setpointis from about 100 Pa to about 1,000 Pa, and the second pressure setpointis from about 0 Pa to about 10 Pa.
 10. The method of claim 9, whereinthe first pressure setpoint is from about 50 Pa to about 100 Pa, and thesecond pressure setpoint is from about 0 Pa to about 10 Pa.
 11. Themethod of claim 2, wherein the turbulence is produced by changing thegas flow rate in the radiation source chamber of a target gas between afirst gas flow rate setpoint and a second gas flow rate setpoint. 12.The method of claim 11, wherein the target gas is hydrogen gas or extradry clean air.
 13. The method of claim 11, wherein the first gas flowrate setpoint and the second gas flow rate setpoint differ by at least0.5 slm.
 14. The method of claim 11, wherein the first gas flow ratesetpoint is from about 0.5 slm to about 1 slm, the second gas flow ratesetpoint is from about 0.1 slm to about 0.5 slm, and the target gas ishydrogen gas.
 15. The method of claim 11, wherein the first gas flowrate setpoint is from about 5 slm to about 10 slm, the second gas flowrate setpoint is from about 0.1 slm to about 5 slm, and the target gasis extra dry clean air.
 16. The method of claim 2, wherein thetemperature, pressure, or gas flow rate is changed over irregular timeperiods.
 17. The method of claim 16, wherein the irregular time periodsindependently range from about 24 hours to about 96 hours.
 18. An EUVlight source having means for changing at least one of the pressure,temperature, or gas flow rate in a radiation source chamber thereof atregular or irregular time periods.
 19. The EUV light source of claim 18,wherein the means for changing is configured to change a temperature ofvanes, or a pump rate of a vacuum pump, or a gas flow rate through gasports.
 20. A method for generating turbulence in a radiation sourcechamber of an EUV light source, comprising: changing at least one of thepressure, temperature, or gas flow rate in the radiation source chamber.